Hard small particles, collectively called “contamination,” are among the most frequent cause of failures in typical hydraulic systems. Contamination is carried by the fluid flowing through the hydraulic system, penetrating virtually every flow passage. To combat this problem, a typical hydraulic system is equipped with a replaceable low-pressure filter that is usually located in the return-to-tank line at the end of the entire hydraulic circuit. The filter's role is to separate and store the maximum possible amount of contamination over multiple passes of the fluid.
Most hydraulic systems use hydraulic valves to control flow parameters and to steer the various flows in desired directions. Their need for precision and tight clearances can make hydraulic valves very sensitive to contamination. So called “two-stage valves” are particularly sensitive to contamination. A two-stage valve is a valve in which the main element is controlled by a smaller flow, called a “pilot flow.” Small flows, such as pilot flows, are typically created by limiting the flow of fluid through a small orifice. If the orifice is exposed to contamination, then the particles that make up the contamination may not pass through and, eventually, may block the orifice. A blocked orifice is a common cause of valve failure or hydraulic system failure.
Most two-stage, or piloted, valves have an additional small filter built inside the valve to protect the pilot flow orifice from blocking. Existing “in-valve” filters are not very effective, however. In most cases, after being exposed to a contaminated fluid an in-valve filter has to be factory cleaned, which involves removing and rebuilding the entire valve. Furthermore, existing in-valve filters tend to be expensive.
Filters that are used for inner passage fluid filtration in high and low pressure hydraulic applications typically operate on a separation by restriction and storage principle. In such filters, fluid is passed through multiple small openings of a filter that is installed in front of an orifice. These filters are usually made from a metal screen or perforated material, and are classified by their percentage of open area.
In general, the more open area a filter has, the better it tends to be. However, in some applications, filters with large open areas are not practical because of space constraints. For applications that are space sensitive, small yet highly open filters may be used. However, the latter type of filter has its own drawbacks, which are most evident in high-pressure applications. One drawback is that the filter element is structurally weak and requires additional support to withstand differential pressure force when the filter is clogged with debris. Another drawback of small, highly open filters is that they tend to permit large particles to enter.